In the process of screen printing, the result achieved by the printing equipment is no better than the screen material utilized therewith. Even if a frame is stable and does not change during operation, inaccurate results can be achieved if the fabric screen material fails or stretches during the printing process.
New low elongation fabrics have been developed in recent years. However, even these fabrics begin to relax soon after the squeegee is first applied to transfer ink during the printing process. This is analogous to the tuning of a guitar with new guitar strings. The process is thus complicated by this initial relaxation during use. Loss in screen tension during any part of the printing process can result in image distortion, mis-registration, blurred edges, color shifts, ink penetration through porous or semi-porous substrates (such as uncoated paper or cloth garments), and ink build-up on the bottom of the screens. Manifestly, these changes in tension will require ink, squeegee and press adjustments during the printing operation. The result of this change in tension is a loss in quality, productivity and control over the printing process.
Roller type frames, such as those manufactured by Stretch Devices, Inc. of Philadelphia, Pa., may be utilized to meet the rigidity and stability requirements for accurate screen printing results. These roller type frames permit the retensioning of the fabric so as to maintain a constant tension over the entire printing operation. In a roller type frame, retensioning is performed by rotating (at least) one roller within the frame. This retensioning is performed after the fabric has received the stress exerted by the squeegee and has reacted to all of the inks and chemicals in the processes. This retensioning tends to restress or shock the polymer chains in the screen fibers and is essentially a work-hardening type process. During retensioning, the molecular chains become more and more highly oriented in the direction of the fibers while developing even greater bond strength. The more the screen is used and retensioned (restressed) after reclaiming, the stronger and more stable it becomes. Manifestly, screens that have been worked for an extended period of time will have consistency and repeatability in their printing.
Higher tensions in a screen can result in a faster squeegee speed during the printing operation. The squeegee speed is typically dictated by the speed at which the screen lifts off the substrate. In the lifting of the substrate, a "snap"-action or force is desired to assist in ink shear. Ink shear is important in order to provide accurate lines during the printing process. High tension gives a quicker and/or more forceful snap of the screen off of the substrate and thus a higher ink shear. This results in the ability to use a faster squeegee speed during printing, and thus a faster printing process.
Higher tensions also result in a more uniform application of the ink on the screen by the flood bar prior to the squeegee moving across the screen. Because of the higher tension, there is no build-up at the center of the screen material due to a downward bowing caused by the weight of the ink on the screen material. Moreover, because of the tension in the screen, the flood bar can move more quickly and at a greater force without resulting in the ink being pushed through to the opposite side of the screen material.
In the operating machinery, the off contact distance between the screen and the substrate can be lowered by using higher tension. The lower the off contact distance, the less force required in order to make contact between the screen and the substrate and the less stretching of the image. Off contact distance is also a consideration in determining the snap force by the screen away from the substrate. In higher tensions, the increased snap force is created by the tension in the screen as opposed to the stretching of the screen at a greater off contact distance.
Another advantage obtained by higher tension is permitting the edge of the image on the screen to be closer to the edge of the screen itself. Manifestly, relatively smaller frames may be used and the stroke length of the flood bar and squeegee can be reduced without affecting the resultant image.
Another advantage of the higher tensions in the screen relates to the consistency of thickness of deposit and to color control. These factors are a result of interface pressure between the screen and the substrate upon the application of the ink. With a high interface pressure, there is a significant likelihood of the spread of the ink on the printing substrate. With a higher tension, the amount of force needed to be applied to the substrate is substantially reduced. However, there is little reduction in the force being applied by the squeegee in causing the ink to pass through the screen material to receive a consistent coverage on the substrate.
Other advantages can also be achieved by a high tension printing process. However, these advantages have heretofore not been achieved at the rate possible with the present invention.